Plasma or serum samples from a large number of vertebrate species were screened for the presence of a papain-binding protein resembling human alph a 2-macroglobulin (alpha 2M). The screening method depended on the unique property of alpha 2M of binding proteinases in such a way that the enzyme retains partial activity against low-molecular-weight substrates. A papain-binding protein was detected in serum from members of all the major vertebrate taxa. In mammals, birds, reptiles and amphibians the protein had an Mr similar to that of human alpha 2M (725 000), but in fish, including dipnoans, actinopterygians, elasmobranchs and cyclostomes, the papain-binding protein was of Mr about 360 000. Of the invertebrate species tested, all of which were arthropods, two were negative, but the horseshoe crab, an arachnid, did possess a papain-binding protein, although this was heterogeneous in electrophoresis and differed from alpha 2M in resisting inactivation by methylamine. From the results, and a detailed study of the properties of the fish papain-binding protein described in an accompanying paper [Starkey, Fletcher & Barrett (1982) Biochem. J. 205, 97-104], it seems that alpha 2M first appeared in an ancestor of all modern vertebrates as a protein of Mr 360 000 and that the larger macroglobulin (Mr 725 000) first appeared in an ancestor of the tetrapods.
